<!doctype html>
<html><head>
<title>Circular border</title>
<style>
body { margin: 0 }
div {
  margin-left: 8px; margin-top: 8px;
  width: 50px; height: 50px; 
  border: 10px solid black; 
  border-radius: 10px;
}
div > div {
  margin: 0; width: 50px; height: 50px;
  border-radius: 0;
  background: black;
  border: none;
}
</style>
</head>
<body><div><div></div></div></body></html>
